GTA: Detecting affected dependent Go packages

The open-source tool gta (Go Test Auto) is used to understand the downstream dependencies of Go packages changed in pull requests to monorepositories like cthulhu. It compares the current branch with its merge base of the destination branch to determine what's been changed and calculates all dependencies of those changes, outputting the import paths of affected packages. Gta dramatically reduces build times by targeting only impacted packages. The tool is now used almost everywhere in DigitalOcean's build pipeline, including static analysis checks, code compilation and testing, and artifact builds and deployment.